A photovoltaic generator is a device that generates electricity from light. There has been a constant effort over the past few years to increase the efficiency of photovoltaic generators. Traditional solar panels, having an array of solar cells, have become efficient to a point that commercialisation is now widespread. Such traditional panels nevertheless have their disadvantages. For example, to maximise power yield, they should be adapted to follow the sun as it crosses the sky. Additionally, the traditional panels do not generate electricity particularly well if the incident light is diffuse, for example on a cloudy day.
In an effort to overcome some of the problems associated with traditional panels, research has recently been carried out into developing photovoltaic generators that can be used in stationary systems and that can operate more efficiently when ambient light is diffuse. One type of photovoltaic generator that has been developed is a luminescent photovoltaic generator. This type of device includes a photovoltaic cell and an associated luminescent portion. The device is typically arranged such that incident light passes into the luminescent portion, at least some of the incident light being absorbed, then emitted and passed to the photovoltaic cell, where the light generates electricity. The luminescent emission of the light is typically in all directions, so that suitable reflectors ideally should be placed around sides of the luminescent portion to minimise loss of emitted light, and to guide as much of the emitted light to the photovoltaic cell. Luminescent photovoltaic generators, although promising in theory, have yet to reach an efficiency that allows them to be used commercially. There is a constant desire to improve efficiency of luminescent photovoltaic generators.
Recent research in the field has been focussed on the use of organic luminescent materials. This is exemplified in a paper by Currie et al in Science, 11 Jul. 2008,
Vol. 321. no. 5886, pp. 226-228, entitled High-Efficiency Organic Solar Concentrators for Photovoltaics. This paper discloses organic luminescent solar concentrators for use in generating electricity with solar cells. The solar concentrators comprise a glass substrate having a thin film of an organic dye on a surface thereof. Two types of organic dye were used: 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), a fluorescent dye, and platinum tetraphenyltetrabenzoporphyrin [Pt(TPBP)], a phosphorescent dye.
The organic luminescent solar concentrators disclosed in the paper mentioned above by Currie et al have fairly broad emission spectra, in common with most organic luminescent materials. With such materials, reflectors on the outside of the luminescent portion of the concentrators need to be able to reflect most, if not all, of the emitted light. On the side of the device on which light is incident, the reflector must allow incident light of an appropriate wavelength through to the luminescent material so that this light can be absorbed, but it must prevent emitted light from the luminescent material passing through the reflector, and instead reflect this, so that it can reach the solar cell. Such reflectors are termed light or wavelength filters. It can be costly to provide light filters that reflect across a broad range of wavelengths.
Organic luminescent materials also suffer from degradation over an extended period, particularly from incident UV light.